Several different types and methods of producing solar cells are known in the industry. An ongoing objective of solar cell manufacturers is to improve the conversion efficiencies of the solar cells in a cost effective manner.
More recently, higher energy conversion efficiencies have been obtained with mechanically stacked multijunction solar cells. This mechanical stacking generally consists of stacking a top cell that absorbs higher frequency light (i.e. a high bandgap cell) on a booster cell which will absorb the lower frequency light that passes through the top cell (i.e. a low bandgap cell). See Fraas, "Current Topics in Phtovoltaics", p. 169, Academic Press (1985) Partain et al., "26.1% Solar Cell Efficiency For Ge Mechanically Stacked Under GaAs", 62 J. Appl. Phys., p. 3010 (1987). One example of a lower band gap booster cell is germanium (Ge). See Partain, supra, Another example of a lower band gap booster is gallium antimonide (GaSb). See Fraas et al, "GaSb Film Grown by Vacuum Chemical Epitaxy Using Triethyl Antimony And Triethyl Gallium Source", 61 J. Appl. Phys., p. 2861 (1987). Theoretical projections of the performance of a GaAs on GaSb mechanical stack have been reported; see Fraas et al, "Near-Term Higher Efficiencies With Mechanically Stacked Two-Color Solar Batteries", 19 Solar Cells p. 73 (1986-87), but no high performance booster cells have previously been fabricated. Copper Indium Diselenide is another booster cell for GaAs. See U.S. Pat. Nos. 4,680,422 and 4,795,501.
In Mc Leod et al U.S. Pat. No. 4,776,893 it was presumed that a passivation window of aluminum gallium antimonide (AlGaSb) would be required for the bottom cell. Forming such a passivation window would necessitate the use of a costly, low throughput epitaxial processing to produce the GaSb cells.
GaSb photodiodes fabricated by zinc diffusion from a zinc-silica spin on film is described by W. Schmidt auf. Altenstadt and C. Heinz in Physica 129B, p. 497, 1985. The zinc concentration obtained by that process were too low for solar cells.